Think of gamma like a bright ball of light inside a dark sphere holding cell:

bright light = radiation
holding sphere = containment vessel

then think of putting a pin prick in the sphere in a dark room.

you would see the light from the prick, and marking a spot on the wall.

gamma ray on clouds.

as we know, you cannot see it without specialised equipment.

this is why fukushima is so scary, remove the containment sphere and the light shines everywhere......